Separation-purification process using an RO membrane system, which consumes less energy than systems that use evaporation or electrodialysis, has been widely used for desalination of seawater or salt water, production of industrial water or ultrapure water, recovery of wastewater, and the like.
Since the fouling of an RO membrane degrades the properties of the RO membrane, RO membranes are periodically cleaned in order to recover the properties of the RO membranes. The development of a cleaning agent and a cleaning process that are further effective for cleaning RO membranes has been anticipated.
For cleaning an RO membrane, for example, acids (e.g., oxalic acid and citric acid), alkalis (e.g., sodium hydroxide), surfactants (e.g., sodium dodecyl sulfate and sodium dodecylbenzenesulfonate), chelating agents (e.g., EDTA), combined chlorine agents, and oxidizing agents, have been used depending on the properties of a foulant that fouls the membrane (Non-patent Literature 1).
The materials of RO membranes recently used are broadly classified into aromatic polyamides and cellulose acetates. Aromatic polyamide RO membranes, which have low resistance to oxidizing agents but high resistance to alkalis, can be cleaned even under an alkaline condition where the pH is 10 or more. In contrast, cellulose acetate RO membranes, which have higher resistance to oxidizing agents (e.g., chlorine) than aromatic polyamide RO membranes but low resistance to alkalis, cannot be cleaned under an alkaline condition where the pH is 9 or more.
Patent Literature 1 proposes a microbiocide for water systems such as paper-making system, the microbiocide including free chlorine, urea, and an alkali. It is not described in Patent Literature 1 that the microbiocide may be used as an agent for cleaning membranes. It is also not described in Patent Literature 1 that urea may prevent RO membranes from being degraded by cleaning agents.
Patent Literature 2 proposes an agent for preventing biofouling which includes a urea derivative that stabilizes halogens. Although it is described in Patent Literature 2 that the antifouling agent may be used for membranes, in Patent Literature 2, urea is used for stabilizing halogens and it is not described that urea may prevent RO membranes from being degraded by cleaning agents.    Patent Literature 1: Japanese Patent No. 5339921    Patent Literature 2: JP 2012-529496 A    Non-patent Literature 1: “Maku-Shori Gijyutsu Taikei (Jyou-kan)” (“Outline of Membrane Separation Technology (Vol. 1)” published by Fuji Techno System) p. 836 (1991)